Robson F. S. Dias

Extra Long-Distance Bulk Power Transmission

This paper presents a solution for bulk ac power transmission over extra long distance based on an unconventional transmission line that is little longer than a half wavelength (λ/ 2+). This solution is very convenient to be applied in countries where the generation sites are very distant from the consumption centers. However, it has some restrictions regarding feeding loads at intermediate points. To overcome this drawback, an HVAC tap based on power-electronics converters is proposed. This tap allows feeding local loads as well as integrating power plants to the main circuit without changing the transmission systems electrical characteristic. This paper also presents an application example of a λ/ 2+ line with an HVAC tap draining and injecting energy from the line.

FACTS for Tapping and Power Flow Control in Half-Wavelength Transmission Lines

This paper presents a solution for transmission of bulk power over long distances. It allows the interconnection between remote power plants and power grids, which is a common situation in countries with continental dimensions, such as China, Russia, Brazil, and Canada, and also special projects like supergrids in Europe and Asia. This paper also presents an analytical analysis of series and shunt FACTS applications for tapping and power flow control in half-wavelength transmission lines. The proposed FACTS devices are able to drain or inject fractional active power in the half-wavelength line and simultaneously to control the main power flow through the line by means of reactive power compensation, without mischaracterizing its particular principle of half-wavelength operation. Simulation results from a frequency-dependent model of transmission line and the complete models of the series and the shunt FACTS devices were obtained in the PSCAD/EMTDC to demonstrate the feasibility of the proposed systems.

Analysis of Noncharacteristic Harmonics Generated by Voltage-Source Converters Operating Under Unbalanced Voltage

This paper deals with an analytical model to evaluate the noncharacteristic harmonics that are generated by three-phase three-wire voltage-source converters (VSCs) operating under unbalanced sinusoidal voltage conditions. This model permits the calculation of the oscillating current on the VSC dc side by considering the unbalanced voltages on its ac side. It can be used to prove that the use of dc voltage control techniques to eliminate VSC dc-side double frequency voltage ripple necessarily creates three-phase currents with undesirable low-frequency harmonic components on the VSC ac side. In this paper, the attention is directed to the instantaneous oscillating real power component at twice the line frequency, rather than to other power components. The noncharacteristic current components analyzed in this paper, such as the fundamental negative sequence and third-order harmonic components of positive and negative sequences are calculated using the proposed model in time-domain simulations. A simple model to calculate the dc-side capacitor to mitigate these noncharacteristic harmonic problems is presented.

Direct voltage control in grids with intermittent sources using UPFC

This paper proposes the use of UPFC for direct voltage control in electric grids with highly intermittent generation sources such as wind generators. The variable characteristics of these sources strongly affect the voltage profile in the grids. The objective of this paper is to show through a conceptual analysis that it is possible to obtain precise and fast voltage control by using an UPFC. The UPFC rating is smaller than that of a STATCOM or SVC, which should be necessary for indirect voltage control. The UPFC does not have limitation due to the number of operation per day as in the case of transformers with on load tap changer. The UPFC based on single-phase VSC are analyzed and the series converter is connected directly to the grid without transformer. It controls the voltage independently in each phase, therefore balancing it, if necessary. Simulation results using PSCAD/EMTDC are presented to confirm all these advantages of the use of UPFC for direct voltage control.

Approximation of Lightning Current Waveforms Using Complex Exponential Functions

A key aspect to evaluate the lightning performance of transmission lines is the correct representation of the lightning current waveform considered. Traditionally, the analytical modeling of such data might involve the use of double-exponential functions or even Heidler functions series, i.e., partial fractions with polynomial functions of variable degree. Herein, we propose the fitting of lightning current waveforms to obtain a pole-residue model as an alternative procedure. Purposely, we investigate the performance of the vector fitting (VF) and time-domain vector fitting (TD-VF) methods to represent continuous lightning current waveshapes originally modeled using Heidler function series. Four nontrivial test cases of single-crest and double-crest peaked lightning waveforms are analyzed. Albeit comparatively accurate results are achieved, the TD-VF provides models with lower pole orders than the VF approach for equivalent window observation times and time steps. Incorporation of pole-residue models in transient simulation programs can be straightforward as compared to a Heidler function series heuristic approach.

Non-characteristic harmonics and DC side capacitor calculation in VSC connected to a distribution system with unbalanced voltage

This paper presents a model of typical three-phase, three-wire and two-level PWM controlled voltage-source converters (VSC) suitable to investigate the non-characteristic harmonic generation at dc- and ac-side of the converter under sinusoidal unbalanced voltage condition, e.g., third order current harmonic component of positive-sequence in the VSC ac-side current. The proposed model, based on instantaneous power as well as switching functions concepts, permits to evaluate the oscillating voltage that appears on the VSC dc-side. The effect of the negative-sequence voltage component is considered on the dc capacitance calculation which consequently enables to size the dc capacitor accurately in order to keep the dc voltage ripple under pre-defined limits. The model also shows the exact dc-side voltage ripple due to the VSC ac-side negative-sequence voltage component. The dc capacitor size and the non-characteristic harmonics obtained based on the proposed model are assessed using time-domain simulation in the PSCAD program.

Synthesis of controlled reactances using VSC converters

This work presents a review of the main techniques for synthesizing positive and negative inductance using power electronics: VAPAR (variable active-passive reactance), BVI (bootstrap variable inductance), AVI (active variable inductance) and DRS (direct reactance synthesis). Based on this review a new topology for synthesizing a fundamental frequency variable reactances based on synchronism circuits (PLL - phase-locked loop) is presented. The main advantage of this new technique is that the control signals are immune to the noise from the grid. Some simulation results, using PSCAD/EMTDC, are presented for the proposed circuit.

POD-PWM applied to circulating current control in HVDC-MMC based system

The Modular Multilevel Converter (MMC) control presents more technical challenges than the regular 2-level Voltage Sourced Converters (VSC) one. One of the most common type of control investigaded in literature is based on techniques such as the Phase Opposite Disposition PWM (POD-PWM), which include balancing capacitor voltages and circulating current control. This paper presents some details that are not clearly explored in the main references about this theme. The control system implementation is described and its performance is evaluated through an HVDC-MMC based system simulation using the Matlab Simulink software. The results show that the circulating current control is more effective if the total number of submodules connected at the same time (positive arm plus negative arm) is not constant, and most control techniques do not take advantage of this. Besides, the results show that the effect on the DC current is negligible.

Experimental-theoretical thermal and electrical analyses of insulated gate bipolar transistors (IGBT) power module

Applications of high-power insulated gate bipolar transistor (IGBT) modules include railway traction, motor drives, and hybrid electric vehicles. The reliability of these semiconductor devices is tightly linked to the operating junction temperatures of IGBT and diode chips present in them. Since these temperatures are very difficult to measure, accurate models and simulation tools are required to compute the instantaneous temperature of the devices under different load conditions. In this paper, we describe a transient 3D heat transfer numerical model of an IGBT power device with many layers of varying cross-sectional areas, distinct materials, and heat sources. Two cases were evaluated according to the total power dissipation considered. In the first case, a non-switching constant conduction scenario was considered in which a power dissipation of 6.15 W based on experiments was adopted and the calculated results were validated against experimental data obtained via infrared thermography, and excellent agreement between the results was observed. For the second case, IGBT switching — along with power losses due to the gate-closing and gate-opening transitions between conducting and non-conducting states — was taken into consideration. For this case, a higher power of 27.23 W was considered to represent the average power dissipation associated with a typical real-life application of the IGBT unit at a switching at frequency of 1 kHz. For this case, the power dissipation on the IGBT chip was obtained from an electrical simulation and used in the heat transfer problem as a strongly time-dependent heat source. The temperature distributions for both cases were then critically compared.

Indirect Voltage Control (IVC) versus Direct Voltage Control (DVC) in distribution grids with renewable energy sources

The voltage control in grids with high intermittent voltage profile, as the distribution grids with renewable energy source, can be based on Indirect Voltage Control (IVC) or Direct Voltage Control (DVC). In the distribution level, there are innumerous issues that influence the voltage profile during the operation of an Intermittent Generator (IG), such as load conditions or system characteristics (for example, line impedance). In this paper, the results of a comparative analysis between IVC and DVC are shown for different grid conditions. This comparison is done in terms of the apparent power that each equipment requires to compensate the voltage at the Point of Common Coupling (PCC) of the generic distribution grid considered. An analytical grid modelling including the IVC and DVC is used to compare both controllers in terms of the apparent power to keep the PCC voltage controlled when different values of current are injected on the grid by an IG. Numeric results are presented to show the best solution in terms of the equipment rating.